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The first 3D images of the dawn of life have been made possible using a new technique allowing virtual dissections of half-billion year old fossil embryos, scientists say.

The images, including those of ancient penis worm embryos, are published today in the journal Nature.

Among the images were those of fossils from the genus Markuelia, found in China and Siberia, which date from the Cambrian period.

The new imaging method reveals a universe of detail impossible using previous methods, from the first splitting of cells to just before hatching.

The researchers, including those from the UK's University of Bristol, say their technique pushes back the frontiers of science much as the scanning electron microscope did half a century ago.

"We are looking at the dawn of life," says lead researcher Dr Phil Donoghue.

"Because of their tiny size and precarious preservation, embryos are the rarest of all fossils.

"But these fossils are the most precious of all because they contain information about the evolutionary changes that have occurred in embryos over the past 500 million years."

Until now, if scientists had wanted to study fossil embryos, they had to look at them from the outside or cut narrow slices of the embryos, which obviously destroys them.

But this new method, known as synchroton-radiation x-ray tomographic microscopy or SRXTM, leaves the tiny fossils untouched, yet gives graphic details of their structure.

Accelerating particles

The researchers used a 500 metre wide particle accelerator in Switzerland to deep-scan the minute fossils.

They then fed the information into a computer to generate complete 3D images of the internal structures in fine detail.

"The best analogy is with a medical CT scan ... but at 2-3000 times the resolution," Donoghue says.

"We can see details less than 1000th of a millimetre in dimension.

"We can look at any and every part of the fossil, inside and out, without harming it and then virtually dissect it however we like."

The team says its discovery could roll back the evolutionary history of arthropods like insects and spiders.

In one case they had found hitherto hidden details of the interior structure of an ancient relative of the living penis worm, and in another they had seen embryonic worm segments unlike those found in living specimens today.